TAS2R38: A Model System for Genotype-Phenotype Studies. Prior studies have identified two main forms of T2R38, active and inactive, which are characterized by three genetic variants in the TAS2R38 gene. These variants result in three amino acid changes, proline (P) to alanine (A) at position 49, alanine (A) to valine (V) at position 262, and valine (V) to isoleucine (I) at position 296 in the T2R38 receptor. Individuals who are homozygous for the active (PAV/PAV) form detect bitterness in compounds containing a thiourea (-N-C=S) moiety, including PTC, 6-n-propylthiouracil (PROP), and the plant compound goitrin, common in foods such as green vegetables (7-9). They also respond to acyl-homoserine lactones (AHLs), a class of compounds produced as signaling molecules by certain bacteria, triggering a rapid defense reaction consisting of increased ciliary beat frequency (CBF) to facilitate mucociliary clearance, and generation of nitric oxide (NO), a gas that can diffuse into the airway and kill bacteria (4). In contrast, those who are homozygous for all three variants (AVI/AVI) consume these compounds without perceiving them as bitter and do not appear to respond to AHLs (10). The frequency of both the active and inactive forms of TAS2R38 is at a near balance of 50:50 in many human racial groups, including Americans of European and African descent.